Formation fluid obtained from a reservoir generally contains a number of natural constituents, such as water, super critical gas, and liquid hydrocarbons. In addition to these natural constituents, the composition of the formation fluid may also include an artificial contaminant such as filtrate including water-based mud or oil-based mud, used during drilling operations.
Constituents of this formation fluid may be identified by sampling the fluid and then conducting an analysis on the composition of the sampled fluid. The analysis is generally performed by making special measurements of the fluid to characterize the composition and as such infer many properties of interest about the fluid. Knowledge of these properties is useful in characterizing the reservoir and in making many engineering and business decisions.
Various tools can be used to perform analysis of downhole fluids. For example, spectrophotometers, spectrometers, spectrofluorometers, refractive index analyzers, and similar devices can be used to analyze downhole fluids by utilizing appropriate sensors to measure the fluid's spectral response. Another type of measurement that can be made on sampled fluid is taking density measurements. For example, density measurements are sometimes made at fixed time intervals, and analyzed to estimate the sample's quality. The repeated density measurements can be used to plot the change in density over time.
Characteristics of this density-time plot can then be used to assess the composition and contamination level of the sampled fluid. Other types of measurements that can be used in characterizing fluid composition include monitoring density, pressure, temperature and the like.
Though various techniques have been used in the past for characterizing fluid composition of a fluid sample, most of these techniques lack the level of accuracy desired and may also be inefficient. For example, a sample that is taken with too high a contaminant level may produce inaccurate or misleading results about the composition of the formation fluid. Historically, the problem is either handled by assuming that the exact properties of the filtrate and the formation are known, in addition to the initial condition, or by simply watching the changing measurement of the mixture and waiting for a measurement plateau that is interpreted as indicative of maximum achievable fluid purity.